Suzaku Wide-band All-sky Monitor view of soft gamma-ray transient
Masanori Ohno1 , Kazutaka Yamaoka2,3 , Makoto S. Tashiro4 , Kevin Hurley5 , Hans A. Krimm6,7 , Amy Y. Lien6,8 ,Norisuke Ohmori9 , Satoshi Sugita10 , Yuji Urata11 , Tetsuya Yasuda4 , Ryuuji Kinoshita9 , Yusuke Nishioka9 ,Yoshitaka Hanabata12 , Jun’ichi Enomoto4 , Takeshi Fujinuma4 , Yasushi Fukazawa1 , Wataru Iwakiri13 ,Takafumi Kawano1 , Motohide Kokubun14 , Kazuo Makishima15 , Shunsuke Matsuoka4 , Tsutomu Nagayoshi4 ,Yujin Nakagawa16 , Souhei Nakaya4 , Kazuhiro Nakazawa17,18 , Takanori Sakamoto19 , Tadayuki Takahashi14 ,Sawako Takeda4 , Yukikatsu Terada4 , Seiya Yabe4 , and Makoto Yamauchi9

1Department of Physics, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
2
Institute for Space-Earth Environmental Research, Nagoya University,Furo-cho, Chikuka-ku, Nagoya, Aichi 464-8601, Japan
3
Division of Particle and Astrophysical Science, Graduate School of Science,Nagoya University, Furo-cho, Chikuka-ku, Nagoya, Aichi 464-8602, Japan
4
Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan
5Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720-7450, USA
6Center for Research and Exploration in Space Science and Technology (CRESST)
and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
7Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA
8Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
9Department of Applied Physics, University of Miyazaki,
1-1 Gakuen kibanadai-nishi, Miyazaki-shi, Miyazaki 889-2192, Japan10
Department of Physics, Tokyo Institute for Technology, 2-12-1
Ookayama, Meguro-ku, Tokyo 152-8550, Japan
11
Institute of Astronomy, National Central University, Chung-Li 32054, Taiwan
12Institute for Cosmic-Ray Research, University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa-shi, Chiba 277-8582, Japan
13
MAXI Team, The Institute of Physics and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
14
Department of High Energy Astrophysics, Institute of Space and Astronomical Science (ISAS),
Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo, Sagamihara 252-5210, Japan
15
RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
16
Science-satellite Operation and Data Archive Unit, Institute of Space and Astronomical
Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku,
Sagamihara-shi, Kanagawa 252-5210, Japan17
Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan18
Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
19
Department of Physics and Mathematics, Aoyama Gakuin University,
5-1-1 Fuchinobe, Sagamihara, Kanagwa 252-5258, Japan
E-mail(MO): ohno@astro.hiroshima-u.ac.jp
Abstract
In this paper, we review results of soft gamma-ray transients observed by the Wide-band All-skyMonitor (WAM) onboard Suzaku satellite. The WAM had been operated for about 10 years from 2005 to2015, and observed various types of transients and bright gamma-ray sources, such as gamma-ray bursts(GRBs), solar flares, soft gamma repeaters (SGRs), gamma-ray emission from Crab nebula ..etc. Thanks
to its very large eﬀective area up to soft gamma-ray energy band (∼400 cm2 even at 1 MeV), we can detect
soft gamma-ray photons from many objects with good statistics. Especially, detected number of GRBs is
more than 1400, which is comparable detection rate to other GRB instruments. With those large numberof samples, we can discuss many kinds of science topics. For example, the duration distribution of observed
GRBs, timing and spectral properties for GRBs, weak GRBs with high redshifts, time-resolved pulses forlong GRBs, spectral properties of solar flares in hard X-ray energy band, and emission mechanism of brightbursts from SGRs. In addition to transient observations, the WAM has the capability to monitor the bright
gamma-ray sources utilizing earth occultation technique. We successfully monitored some sources such as
Crab, Cyg X-1 from 150 to 500 keV energy band and we can trace the broad-band spectral variability.
Key words: Suzaku:HXD-WAM — all-sky observations

The Hard X-ray Detector (HXD(Takahashi et al.
2007)), which is one of two narrow-field instruments onboard Suzaku satellite, is surrounded by very large 20BGO crystals. The primary role of these BGO crystals
is to serve main detector as an active shield. In addition,
their large geometrical, eﬀective area and wide field ofview enable us to observe all-sky covering very wide energy band (Wide-band All-sky Monitor; WAM(Yamaokaet al. 2017)). The WAM has the largest eﬀective area
above 300 keV among any other all-sky instruments asshown in Fig 1.

Fig. 1. Comparison of eﬀective area with other all-sky instruments

The WAM has been operated for ∼ 10 years from August 2005 to May 2015. There were 2935 onboard triggers throughout the mission, consisting of 856 GRBs confirmed by other instruments, 309 possible GRBs judged
from the light curve, 48 SGRs, 81 solar flares and so on.
Total detected number of GRBs, including un-triggered
events is more than 1400 and an average detection rateis ∼140 GRBs per year. Utilizing such large numberof GRB samples and thanks to the large eﬀective areaof the WAM, detail studies for spectral, temporal, andmulti-wavelength properties of GRBs have been reported
through more than 30 papers.
Spectral parameters of 193 GRB samples reported
in the GCN circulars was compared by Yamaoka etal. (2017) and found that the WAM Epeak distribution
has a mean around 700 keV, much larger than that of
Swift/BAT and Fermi/GBM. This is due to instrumental
eﬀect that the low energy coverage of the WAM is slightly
higher than other instruments. Krimm et al. (2009)
confirmed that BAT and WAM joint analysis shows themean Epeak value of around 250 keV, consistent withother results. The Epeak evolution for both individual
and inside pulses are studied by Sugita (2009) and Ohno
et al. (2009) and they found the clear correlation between time-resolved Epeak and isotropic luminosity Liso .
Based on synchrotron shock scenario, this suggests constant emission radius. The WAM also observed high-zGRBs, GRB 050904(z=6.3), GRB 120521C(z∼6.0), andGRB 130606A(z∼5.9), and found that they also follow
the same Epeak − Eiso correlation as normal GRBs.
One of the most important temporal property to dis-cuss its origin is the duration distribution. The detail study using WAM sample has been reported byOhmori et al. (2016) and confirmed clear bimodal,
short/long GRB separation, which is similar to that ofINTEGRAL/SPI-ACS and BATSE. The origin of differences from other instruments such as Swift/BAT isalso discussed. Tashiro et al. (2012) studied the temporal decay properties of GRBs and revealed that theenergy dependence of the decay constant could be explained by the synchrotron emission model. Combining
to other wavelength information of afterglow emission
allows us to estimate jet-collimation corrected enegetics.
Urata et al. (2009, 2012) examined the correlation between jet-collimation corrected energy Eγ and Epeak andfound an outlier for GRB 071010B. While, Fermi-LAT
detected GRBs are found to be consistent with classicallong GRBs despite of their energetic GeV photon emitting property. Larger sample could reveal a detail of suchoutlier and relation to GeV emission property.
The WAM also observed many non-GRB transients.
Yasuda et al. (2015) reported that the detection of MeV
photons from outburst of SGR J1550-5418 on January
22th 2009. The spectrum is represented by black bodyplus bremsstrahlung by optically thin plasma or cut-oﬀ
power-law model. Such high energy photon could reveal
environment properties of SGRs. The overall properties
of WAM solar flares is discussed in catalog papers (Endoet al. 2010, Yabe et al. in this volume). In addition, detail spectral properties is also reported for joint analysis
with RHESSI by Ishikawa et al. (2013), and accelerated
electron properties are discussed.
The WAM also has a capability to monitor bright
gamma-ray sources utilizing earth occultation technique.
Some bright gamma-ray sources such as Crab and Cyg
X-1 are successfully monitored for more than 7 years,
and time-averaged Crab spectrum is detected up to 700
keV thanks to the large eﬀective area of the WAM. The
spectrum is consistent with other energy band obtainedby the main detector of HXD. The long-term light curveof the Crab is also consistent with other instruments suchas Fermi/GBM. More detail result of the earth occultation observations will be published.
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